In DEP (Direct Electrostatic Printing) the toner or developing material is deposited directly in an imagewise way on a receiving substrate, the latter not bearing any imagewise latent electrostatic image. The substrate can be an intermediate endless flexible belt (e.g. aluminium, polyimide etc.). In that case the imagewise deposited toner must be transferred onto another final substrate. Preferentially the toner is deposited directly on the final receiving substrate, thus offering a possibility to create directly the image on the final receiving substrate, e.g. plain paper, transparency, etc. This deposition step is followed by a final fusing step.
This makes the method different from classical electrography, in which a latent electrostatic image on a charge retentive surface is developed by a suitable material to make the latent image visible. Further on, either the powder image is fused directly to said charge retentive surface, which then results in a direct electrographic print, or the powder image is subsequently transferred to the final substrate and then fused to that medium. The latter process results in an indirect electrographic print. The final substrate may be a transparent medium, opaque polymeric film, paper, etc.
DEP is also markedly different from electrophotography in which an additional step and additional member is introduced to create the latent electrostatic image. More specifically, a photoconductor is used and a charging/exposure cycle is necessary.
A DEP device is disclosed in e.g. U.S. Pat. No. 3,689,935. This document discloses an electrostatic line printer having a multi-layered particle modulator or printhead structure comprising:
a layer of insulating material, called insulation layer; PA1 a shield electrode consisting of a continuous layer of conductive material on one side of the insulation layer; PA1 a plurality of control electrodes formed by a segmented layer of conductive material on the other side of the insulation layer; and PA1 at least one row of apertures.
Each control electrode is formed around one aperture and is insulated from each other control electrode.
Selected potentials are applied to each of the control electrodes while a fixed potential is applied to the shield electrode. An overall applied propulsion field between a toner delivery means and a receiving member support projects charged toner particles through a row of apertures of the printhead structure. The intensity of the particle stream is modulated according to the pattern of potentials applied to the control electrodes. The modulated stream of charged particles impinges upon a receiving member substrate, interposed in the modulated particle stream. The receiving member substrate is transported in a direction orthogonal to the printhead structure, to provide a line-by-line scan printing. The shield electrode may face the toner delivery means and the control electrode may face the receiving member substrate. A DC field is applied between the printhead structure and a single back electrode on the receiving member support. This propulsion field is responsible for the attraction of toner to the receiving member substrate that is placed between the printhead structure and the back electrode.
A DEP device is well suited to print half-tone images. The densities variations present in a half-tone image can be obtained by modulation of the voltage applied to the individual control electrodes. In most DEP systems large printing apertures are used for obtaining a high degree of density resolution (i.e. for producing an image comprising a high amount of differentiated density levels).
For text quality, however, a high spatial resolution is required. This means that small printing apertures must have to be made through said plastic material, said control electrodes and said shield electrode.
Providing printing apertures in a DEP printhead structure comprising two electrodes (control electrode and shield electrode) separated by an insulating plastic material, to yield a printhead capable of producing images with high resolution and also with uniform density pattern is not an obvious process.
All printing apertures in the printhead structure must have exactly the predetermined diameter, the electrodes must stay in place and have a well defined and constant shape, and the walls of the printing apertures through the insulating plastic must be smooth to avoid clogging of the printing apertures. After forming the printing apertures in the printhead structure, each aperture must be individually addressable such as to be able to yield any density between zero and maximum density. Moreover every printing aperture has to be addressable to the same extent in order to yield smooth density pattern.
There is still a need for a DEP system, using a printhead structure comprising two electrodes (control electrode and shield electrode) separated by an insulating plastic material and wherein printing apertures are present, wherein the printing apertures are not easily clogged by the toner particles and wherein each aperture is individually addressable in a reproducible way.